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RF choke on a video camera--will it remove mains hum?

Discussion in 'Radio and Wireless' started by dietermoreno, Mar 13, 2013.

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  1. dietermoreno


    Dec 30, 2012
    I have another whimsical question.

    I have a Flip cam video camcorder, which is known for having terrible audio quality.

    I looked at the mic and I noticed that it is unshielded and there was no foam padding to equalize the response to bass (bass clips really bad right now).

    It also picks up mains hum. I'm not sure what the mic is, probably an electret, but as long as there are amplification components, we have learned in my guitar amp troubles that a transistor or tube can rectify RF into audio, which here is rectifying mains hum of guitar amp mains power supply transformers at a live performance.

    If I was to wrap some wire labled as stereo wire at Walmart (what I found on the internet to be the correct gauge for building an RF choke) for 13 coil turns (that is the number I found on the internet) with a little antenna sticking out at each end to attract RF to the coils and be attenuated passing through the coils, would it work winding the coils around the camcorder?

    What would be the best location to wind the coils around the camcorder? Would the best location be to have the center of the coil winding (coil number 7) over the center of the mic so that the maximum RF attenuation would be above the mic?

    Then I would secure the coils with duck tape then I would wrap a layer of foam around the camcorder with a hole for the lens, a hole for the battery access, and a hole for the USB port.

    So my objectives are:
    (1) Eliminate picking up RF noise such as mains hum,
    (2) Eliminate bass clipping,
    (3) Eliminate feedback pickup,
    (4) Eliminate picking up audio noise that is not in front of the mic.

    Can I do it?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    A coil around the object will not be effective in removing rf interference. For this purpose use a shield, e.g. a wire mesh or copper foil.

    Mains hum at 50 Hz or 60 Hz has way too low a frequency to be attenuated by a conventional shield. You will need material with high permeability, like e.g. µ-metal.

    To eliminatethe audible hum you should first locate the source. Where does the hum enter the audio chain? IOs it the mic, the amp, or the A/D section? Then you should try to improve the resepctive component's behaviour. E.g. add filters to the supply voltage, add shielding, use a better mic etc. The latter could also improve on directionality and bass pickup.
    I suspect that's out of the question for the Flip type camcorder.

    Next best alternative: get a real camcorder.
  3. dietermoreno


    Dec 30, 2012
    Okay I see in this link that high permeability materials are materials such as nickle and iron.

    So even if you live in a bunker 20 feet underground with DC power and no devices using AC, short of lining your bunker with nickle and iron, mains hum will still enter if there are power lines nearby.

    So the longer wavelength of mains hum allows it to be ground penetrating and penetrates conventional shielding.

    So the best way to shield an electronics device from mains hum would be to in case it in a nickle and iron lined box with inch thick walls.

    So for a pocket sized electronic device, read as its impossible to use shielding to remove mains hum and you can't get good quality with something that can fit in your pocket.

    So I imagine that any filters to remove mains hum would be useless when the electronics device is not shielded in inch thick walls of nickle and steel, like guitar amp is shielded with an inch thick wall so humbucker pickups and filters will work to attenuate mains hum from being picked up by a guitar amp since it is shielded.

    So best advice: Give up and buy a real camcorder?

    Maybee a real camcorder will be shielded with a high permiability material (at the cost that it will no longer be able to fit in my pocket so my boot leg concert recording days are over and now I have to pay the venue to record the concert which I can't afford so I should just give up with concert recordings and leave it to the pros who can afford to), will have a humbucking microphone, and will have filters to remove mains hum?
    Last edited: Mar 14, 2013
  4. BobK


    Jan 5, 2010
    Or maybe it will have a better circuit that does not pick up hum.

  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    I second Bob. A typical camcorder isn't exactly a heavyweight. Therefore no heavy high permeability material inside. Also look at typical radio receivers. Their housings are made from plastic. No magnetic shielding at all.
    The Flip camcorder is either constructed very badly of the source of the mains hum isn't over the air. Do you use a battery or a mains powered supply? Chances are soo much higher that the hum is introdcued via the power supply.
  6. dietermoreno


    Dec 30, 2012
    The Flip camcorder is a hand held device, so it is obviously battery powered.

    Yeah I was wondering how radio receivers, even my cell phone, can be only encased in plastic and not pick up all of the **** mains hum.

    Do radio receivers and cell phones that are not encased in high permiability materials use circuitry similar to humbucker pickups in guitars that attempts to not pick up mains hum?

    Yes the Flip camcorder is probably constructed very badly because it only cost me $20.

    What do you expect for $20.
    Last edited: Mar 22, 2013
  7. dietermoreno


    Dec 30, 2012
    I wonder if a 100K resistor would help, as it does in the crystal receiver to remove background noise.

    but I'm not splitting open my camcorder any time soon because knowing me I'll probably cut the mother board in half in the process of trying to add in a resistor.

    like in this schematic for a noise gate I see that the circuit at its essence with out all the "fancy stuff" (like RC filters, gain controls, and level controls) is simply two 120K resistors connected in series with an OP Amp for one level setting, a 390k resistor connected in series with an OP Amp for another level setting, and a 750k resistor connected in series with an OP Amp for another level setting.

    Maybe it will be a fun project to see if a resistor and an OP Amp will remove the hum from a single coil pickup guitar.
    Last edited: Apr 5, 2013
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010

    You still don't understand the purpose of the resistor in the crystal radio either.

    I think this thread is long past its use-by date.
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